4 runoff and floods

Unit IV
RUNOFF
1
Prof. Pradeep T. Kumawat
BE Civil, ME Geo-Tech.
(Assistant Professor)
Late G. N. Sapkal College of Engineering, Nashik,
Maharashtra, India.

Objective
 Definition of runoff
 Runoff process
 Surface runoff
 Classification of runoff
 Factors affecting runoff
 Methods of Estimating runoff
 Summary of Rainfall-Runoff process
 Base Flow Seperation
 Unit Hydrograph

Introduction:
 Stream flow representing the runoff phase of the
hydrological cycle is the most important basic data for
hydrologic studies.
 Its occurrence and quantity are dependent on the
characteristics of the rainfall event, i.e. intensity,
duration and distribution.
 Runoff can be defined as the portion of the
precipitation that makes it’s way towards rivers or
oceans etc., as surface or subsurface flow.
 Surface runoff can be generated either by rainfall,
snowfall, rainstorms or by the melting of snow, or
glaciers.
3

4
Runoff is that portion of the rainfall or irrigation water
which leaves a field either as surface or as subsurface
flow.
When rainfall intensity reaching the soil surface is less
than the infiltration capacity, all the water is absorbed
in to the soil.
As rain continues, soil becomes saturated and
infiltration capacity is reduced, shallow depression
begins to fill with water, then the overland flow starts as
runoff.

5
Surface detention / Detention storage:
The amount of water on the land surface that can form a
film of water or in transit towards stream channels is
called detention storage or surface detention.
Surface Flow:
Surface flow is water that has remained on the surface and
moves as overland or channel flow.
The surface runoff process:
As the rain continues, water reaching the ground surface
infiltrates into the soil until it reaches a stage where the rate
of rainfall (intensity) exceeds the infiltration capacity of the
soil.
Thereafter, surface puddles, ditches, and other depressions
are filled with water (depression storage) and after that
overland flow as runoff is generated.

6
Flooding: Flooding occurs when a watercourse is unable to
convey the quantity of runoff flowing downstream. Floods can
be both beneficial to societies or cause damage.
Importance of Runoff:
 water balance calculation
 Irrigation scheduling
 The magnitude of flood flows to enable safe disposal
of the excess flow.
 The minimum flow and quantity of flow available at various
seasons.
 The interaction of the flood wave and hydraulic structures,
such as levees, reservoirs, barrages and bridges.

Types of Runoff
Types of Runoff:
There are three major types of runoff depending on the
source:
1. Surface runoff
2. Sub-surface runoff or Interflow
3. Base flow

8
a. Surface Runoff:
That portion of rainfall which enters the stream
immediately after the rainfall.
It occurs when all loses is satisfied and rainfall is still
continued and rate of rainfall [intensity] in greater
than infiltration rate.
b. Sub-Surface Runoff:
That part of rainfall which first leaches into the soil
and moves laterally without joining the water table, to
the stream, rivers or ocean is known as sub-surface
Runoff. It is usually referred is inter-flow.

9
c. Base flow:
It is delayed flow defined as that part of rainfall, which
after falling on the ground the surface, infiltrated into
the soil and meets to the water table and flow the
streams, ocean etc.
The movement of water in this is very slow.
Therefore, it is also referred a delayed runoff.
Total runoff = Surface runoff + GW Base flow.

10
Factors Affecting the Runoff:
Runoff rate and volume from an catchment area or
drainage basin are mainly influenced by following
factors:
1. Precipitation characteristics
2. Shape, size & location of the catchment
3. Characteristics catchment surface
4. Topography
5. Geological characteristics
6. Meteorological characteristics
7. Storage characteristics.

11
1. Precipitation characteristics:
A precipitation which occurs in the form of rainfall starts
immediately as surface runoff depending upon rainfall intensity
while precipitation in the form of snow does not result in surface
runoff.
If the rainfall intensity is greater than infiltration rate of soil then
runoff starts immediately after rainfall. While in case of low
rainfall intensity runoff starts later. Thus high intensities of
rainfall yield higher runoff. It has great effect on the runoff.
Duration of Rainfall: It is directly related to the volume of
runoff because infiltration rate of soil decreases with duration of
rainfall. Therefore, medium intensity rainfall even results in
considerable amount of runoff if duration is longer.

12
Rainfall Distribution:
Runoff from a watershed depends very much on the
distribution of rainfall. It is also expressed as
“distribution coefficient”. Near the outlet of watershed,
runoff will be more.
Direction of Prevailing Wind:
If the direction of prevailing wind is same as drainage
system, it results in peak low. A storm moving in the
direction of stream slope produce a higher peak in
shorter period of time than a storm moving in opposite
direction.

13
2. Shape, size & location of the catchment:
Size of Watershed: A large watershed takes longer time for
draining the runoff to outlet than smaller watershed and vice-
versa.
Shape of Watershed: Runoff is greatly affected by shape of
watershed. Shape of watershed is generally expressed by the
term “form factor” and “compactness coefficient”.
Form Factor = Ratio of average width to axial length of
watershed.
Compactness Coefficient: Ratio off perimeter of watershed to
circumference of circle whose area is equal to area of
watershed.

14
Two types of shape:
A) Fan shape [tends to produce higher runoff very
early]
B) Fern shape [tend to produced less runoff]
Slope of Watershed: It has complex effect. It controls
the time of overland flow and time of concentration of
rainfall. E.g. sloppy watershed results in greater runoff
due to greater runoff velocity and vice-versa.
Orientation of Watershed: This affects the evaporation
and transpiration losses from the area. The north or south
orientation, affects the time of melting of collected snow.

A] Fan shaped catchment:
 All the tributaries are
approximately of the same
size.
 Gives greater runoff because
the peak flood from the
tributaries is likely to reach
the main stream
approximately at the same
time.

B] Fern leaf catchment:
 The tributaries are generally of
different lengths and meet the
main stream at the regular
intervals.
 Such a narrow catchments the
peak flood intensity is reduced
since discharges are likely to be
distributed over a long period of
time.

17
3. Characteristics catchment surface:
Land Use: Land use and land management practices
have great effect on the runoff yield. E.g. an area with
forest cover or thick layer of mulch of leaves and
grasses contribute less runoff because water is absorbed
more into soil.
The runoff also depends upon surface condition of the
catchment which may be cultivated or natural.
Soil moisture: Magnitude of runoff yield depends upon
the initial moisture present in soil at the time of rainfall.
If the rain occurs after along dry spell then infiltration
rate is more, hence it contributes less runoff.

18
4. Topographic characteristics:
It includes those topographic features which affects
the runoff. E.g. inclination or slope of catchment also
upon whether the catchment area is smooth or rugged
terrain. Undulating land has greater runoff than flat
land.
Drainage Density:
It is defined as the ratio of the total channel length [L]
in the watershed to total watershed area [A]. Greater
drainage density gives more runoff.
Drainage density = L/A

5. Geological characteristics:
 Soil type: In filtration rate vary with type of soil. So
runoff is great affected by soil type.
 It is one of the important factor.
 It includes the type of surface soil, subsoil, type of
rock and their permeability characteristics.
 If the soil & subsoil is porous, seepage will be more,
resulting in reduction of runoff or peak flood.
 If the surface is rocky or impermeable then
absorption will be nil which resulting more runoff.
 If rocks have fissures, are porous in nature, have
lava funnels water will be lost resulting less runoff.

6. Meteorological characteristics
 Runoff may also be affected by temperature,
wind speed and humidity.
 If temperature is low and ground is saturated
then runoff will be greater.
 If temperature is high and greater wind velocity
give rise to greater evaporation loss and resulting
in less runoff.
 Other factors such as temperature wind velocity,
relative humidity, annual rainfall etc. affect the
water losses from watershed area.

21
7. Storage Characteristics:
a. Depressions in Ponds, lakes and pools
b. Capacity of the reservoir
c. Stream or Channels
d. Check dams in gullies
e. Flood moderation
f. Upstream reservoirs or tanks.
g. Ground water storage in deposits/aquifers.
The artificial storage such as dams, weirs etc. and
natural storage such as lakes, ponds etc. tend to
reduce the peak flow.

22
Measurement of Runoff:
River discharge, the volume flow rate through a river cross
section, is perhaps the most important single hydrologic quantity.
Measurements of river discharge are required for flood hazard
management, water resource planning, climate and ecology
studies and compliance with transboundary water agreements.
The discharge (or stream flow) of a river relates to the volume
of water flowing through a single point within a channel at a
given time.
Understanding this information is essential for many important
uses across a broad range of scales, including global water
balances, engineering design, flood forecasting, reservoir
operations, navigation, water supply, recreation and
environmental management.

METHODS OF ESTIMATING RUNOFF
Stream flow measurement techniques can be broadly
classified into two categories as:
1. Direct determination
2. Indirect determination
(Notes: Direct measurement of runoff is already explained in unit 1
stream gauging.)

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1. Direct determination of stream discharge:
(a) Area- Velocity Method
(b) Dilution techniques
(c) Moving Boat Method
(d) Electromagnetic method
(e) Ultrasonic method.
2. Indirect determination of stream flow:
(a) Slope-area method
(b) Empirical Formula
(c) Infiltration Method
(d) Rational Method
(e) Unit Hydrograph Method
(f) Hydraulic structures, such as weirs, flumes and
gated structures.

By Empirical Formulae,
tables
In the past, many empirical formulae have been developed, but
these are applicable only to the region where they were
derived. Further more, attention must be given in their
application if the characteristics of the region have been subjected
to manmade disturbance (e.g., settlement, construction activity,
land use in irrigation). These are essentially rainfall-runoff
relationships with additional third or fourth parameters to account
for climatic or catchment characteristics. Some of the important
empirical runoff estimation formulae used in various parts of The
India are given below:
(Notes: Slope area method and hydraulic structure's method of Indirect
measurement of runoff is already explained in unit 1 stream gauging.)

tables
1. Binnie’s Percentage Method: Sir alexander Binnie
(1869) after carrying out experiments on the river in the M.P.
has established following relation.
Annual
rainfall in
mm
500 600 700 800 900 1000 1100
Runoff in
percentage
15 21 25 29 34 38 40

tables
2. Runoff Coefficient: Runoff is a function of rainfall as a
equation in the form R = kP
Where, K is a constant and depends upon the type of surface.
Sr. No. Types of surface Value of constant (k)
1 Urban residential 0.2 to 0.3
2 Commercial & Industrial 0.9
3 Parks, Farm etc. 0.05 to 0.30
4 Concrete or asphalt pavement 0.85 to 1.00

tables
3. Barlow’s Table: T. G. Barlow the first Chief Engineer of
the Hydro-Electric Survey of India (1915) after carrying out
experiment on catchment below 130 square km area in U.P.
Class Description of catchment area % of runoff
A Flat, cultivated & B. C. Soil 10
B Flat partly cultivated soil 15
C Average type 20
D Hills & planes with little cultivation 35
E Very hilly, steep with no cultivation 45

tables
4. Strange’s Tables: W. L. Strange (1892) after carrying
out experiment in Maharashtra has established ratios between
rainfall and runoff.

tables
5. Inglis and DeSouza formula: As a result of careful
stream gauging in 53 sites in Western India, Inglis and
DeSouza (1929), they recommended the two regional
formulae.
a) For plain areas: R = (P/254) × (P-17.8)
b) For ghat areas: R = 0.85 P – 30.5
Where, R and P represent average annual runoff and rainfall
in mm

tables
6. Laceys formula: According to Lacey,
Sr.
No.
Type of
monsoon
Recommended value of (F/S) of catchment
A B C D E
1 Very short 2 0.83 0.5 0.23 0.14
2 Standard length 4 1.67 1 0.58 0.28
3 Very long 6 2.50 1.5 0.88 0.43

tables
7. A. N. Khosala’s Formulae: A. N. Khosala (1960) analyzed
the rainfall, runoff and temperature data for various catchments
in India and USA to arrive at an empirical relationship between
runoff and rainfall. The time period is taken as a month. His
relationship for monthly runoff is:
R = P (T/2.08)
Where, R & P are in cm and T is in °C

By Infiltration Method
Surface runoff = Rainfall - infiltration

Infiltration Indices
P - R - Ia
Windex = --------------------
tr
Where,
P = Total Precipitation in cm
R = Total storm runoff in
cm
Ia = Initial loss
Tr = Rainfall duration in hrs

By Rational Methods
Consider a rainfall of uniform intensity and very long duration
occurring over a basin.
In this method, runoff and rainfall are correlated by following
equation:
Q = C · i· A
Where, Q = Flood flow in cubic meters per second
A = Drainage area that contributes to run off (km2)
i = Intensity of rainfall in cm per hour
C = Coefficient of runoff depend upon catchment characteristics.

Unit Hydrograph Method
Hydrographs: A hydrograph is a graph displaying some property
of water flow, such as stage (i.e. water level), discharge, velocity,
etc., versus time.
For displaying runoff characteristics of a watershed, the
hydrograph is one of discharge (cubic meter per second) versus
time (hours).
It represents watershed runoff at a certain point in the flow and
includes only the rainfall upstream of the point in question.
After determining the infiltration index and unit hydrograph from
the rainfall runoff observation, the flood hydrograph for a given
rainfall excess can be calculated.

HYDROGRAPH
• There are three basic parts to the hydrograph:
(1) the rising limb or concentration curve
(2) the crest segment, and
(3) the recession curve or falling limb or depletion curve
Such hydrographs are commonly used in the design of sewerage,
more specifically, the design of surface water sewerage systems
and combined sewers.

HYDROGRAPH
Components of hydrograph:
 Rising limb: The rising limb of hydro graph, also known as
concentration curve, reflects a prolonged increase in discharge
from a catchment area, typically in response to a rainfall event
 Recession (or falling) limb: The recession limb extends from
the peak flow rate onward. The end of storm flow (aka quick
flow or direct runoff) and the return to groundwater-derived
flow (base flow) is often taken as the point of inflection of the
recession limb. The recession limb represents the withdrawal
of water from the storage built up in the basin during the
earlier phases of the hydrograph.

HYDROGRAPH
 Peak discharge: The highest point on the hydro graph when
the rate of discharge is greatest.
 Lag time: The time interval from the center of mass of rainfall
excess to the peak of the resulting hydrograph.
 Time to peak: The time interval from the start of the resulting
hydro graph.
 Discharge: The rate of flow (volume per unit time) passing a
specific location in a river or other channel.

The concept of unit hydrograph first suggested by Sherman in
1932 is of immense use in the prediction and estimation on of
the flood hydrographs of known rain storm from a catchment.
A unit hydrograph of a catchment is defined as a hydrograph of
direct run off (i.e. the total runoff minus base flow) resulting
from one centimeter of effective rainfall of a specified interval
occurring uniformly over the entire catchment area at a uniform
rate.

The basic assumption of the theory of Unit hydrographs
(U.H.G.) states that if two identical storms occur over a
catchment with exactly identical condition prior to the rain, the
hydrographs of runoff resulting from these two storms would be
expected to be the same.
The important characteristic of unit hydrograph is its specified
duration i.e. a 6 hours unit hydrograph implies a unit hydrograph
resulting from a rainfall of 6 hours duration i.e. it is a hydrograph
obtained by surface runoff from a storm of 6 hours duration that
results in a rainfall excess of one centimeter depth.

Utility of unit hydrograph:
 It enables to estimate the maximum flood discharge of a
given stream or channel.
 It assists in the preparation of a flood hydrograph for any
anticipated rainfall in the catchment.
 The Unit Hydrograph (UH) technique is widely used for
runoff estimation, especially for determining peak
discharges.

Basic Assumption Of Unit Hydrograph:
1) The effective rainfall is uniformly distributed within its duration.
2) The effective rainfall is uniformly distributed over the whole
drainage basin.
3) The base duration of direct runoff hydrograph due to an effective
rainfall of unit duration is constant.
4) The ordinates of DRH are directly proportional to the total amount
of Direct Runoff of each hydrograph (i.e. linear).
5) For a given basin, the runoff hydrograph due to a given period of
rainfall reflects all the combined physical characteristics of basin
(time-invariant).

Construction of a Unit Hydrograph:
1. From the past rainfall records select an isolated intense storm-rainfall of
specific or unit duration. For this storm using isohyetal or Thiessen polygon
method calculate average depth of precipitation over the drainage basin.
2. Using (SRRG) self recording rain gauge data of all the available stations
plot mass curves of rainfall for this storm and obtain, average mass curve of
rainfall. From the average mass curve of rainfall construct hyetograph.
3. To construct hyetograph incremental rainfall quantities during successive
units of time are obtained from the mass curve. The average depths of
rainfall per unit of time are then plotted on ordinate against time as abscissa.
4. Using stage hydrograph and stage-discharge relationship obtain a
complete discharge hydrograph at the drainage outlet for the selected storm.
5. If the recession limb is not smooth and contains bumps, make recession
limb smooth or normal by removing the bumps.
6. Separate the base flow from total storm hydrograph using suitable
empirical method and Subtracting the base flow components plot and obtain
ordinates of direct runoff hydrograph.

7. Either by planimeter or by mathematical calculations find out the area of
the catchment or drainage basin.
8. Either by planimeter or by mathematical calculations find out the volume
of the direct runoff.
Volume of direct runoff = Area within the hydrograph
= ∑ Ordinates × t × (60 × 60) = ∑0 × t
Where,
∑ Ordinates = Sum of the ordinates of direct runoff hydrograph at equal time
interval
t = Time interval between successive ordinates
To get volume of runoff in m3, ordinates have to be converted in cumec and
time interval in seconds.

This volume can be converted into cm of runoff by dividing the same by the area of
the drainage basin sq. m x 100.
9. The duration of effective rainfall of the storm of specific or unit duration is
determined by drawing a horizontal line on the hyetograph in such a way that the
area of the hyetograph above the horizontal line is equal to the volume of direct
runoff. Obviously the area below the horizontal line gives the abstractions. This is
an arbitrary method. For elaborate analysis infiltration indexes and curves will have
to be used.
10. Measure the ordinates of direct runoff hydrograph. Divide these ordinates of
direct runoff hydrograph by the obtained depth of runoff in cm to get ordinate of
unit hydrograph.
Mathematically,
Ordinate of direct runoff hydrograph = Ordinate of direct runoff
hydrograph/Depth of direct runoff in cm
11. Plot these ordinates against uniform and the same time interval as the one used
in direct runoff hydrograph to get unit hydrograph.

Factors Affecting Shape of
hydrograph
The Shape of the hydrograph is influenced by various factors
such as:
• A circular shaped drainage basin leads to rapid drainage
whereas a long drainage basin will take time for the water to
reach the river.
• Topography & relief (e.g. slope, inclination, hilly, Rock type)
• Initial losses such as interception, infiltration, soil moisture etc.
• Variation in the direction and Heavy Storms
• Duration and intensity rainfall
• Duration and intensity Snowfall
• Vegetation cover

METHODS OF BASE
FLOW SEPARATION
The surface-flow hydrograph is obtained from the total storm
hydrograph by separating the quick-response flow from the slow
response runoff. It is usual to consider the interflow as a part of
the surface flow in view of its quick response. Thus only the base
flow is to be deducted from the total storm hydrograph to obtain
the surface flow hydrograph.
There are three methods of base-flow separation that are in
common use.
1. By straight Line Method
2. By Extension of base curve
3. By Backward Extension of base flow

METHODS OF BASE
FLOW SEPARATION
1. By straight Line Method: In this method the separation of
the base flow is achieved by joining with a straight line the
beginning of the surface runoff to a point on the recession limb
representing the end of the direct runoff.

METHODS OF BASE
FLOW SEPARATION
2. By Extension of Base Curve: In this method the base flow
curve existing prior to the commencement of the surface runoff is
extended till it intersects the ordinate drawn at the peak (point C in
Fig. 2). This point is joined to point B by a straight line. Segment AC
and CB demarcate the base flow and surface runoff. This is probably
the most widely used base-flow separation procedure.

METHODS OF BASE
FLOW SEPARATION
3. By Backward Extension of base flow: In this method the
base flow recession curve after the depletion of the flood water is
extended backwards till it intersects the ordinate at the point of
inflection (line EF in Fig. 23.3). Points A and F are joined by an
arbitrary smooth curve. This method of base-flow separation is
realistic in situations where the groundwater contributions are
significant and reach the stream quickly.

DIRECT RUNOFF
HYDROGRAPH
The flood hydrograph obtained after the separation of base flow is
called as direct runoff hydrograph.

EFFECTIVE RAINFALL
Effective rainfall (also known as Excess rainfall) (ER) is that part
of the rainfall that becomes direct runoff at the outlet of the
watershed. It is thus the total rainfall in a given duration from
which abstractions such as infiltration and initial losses are
subtracted.
For purposes of correlating DRH with the rainfall which
produced the flow, the hyetograph of the rainfall is also pruned by
deducting the losses. The initial loss and infiltration losses are
subtracted from it. The resulting hyetograph is known as effective
rainfall hyetograph (ERH). It is also known as excess rainfall
hyetograph.

Part of the rain water is lost through deep percolation and run off

56
Determination of runoff coefficients:
The runoff coefficient from an individual rainstorm is defined
as runoff divided by the corresponding rainfall both expressed
as depth over catchment area (mm):
Run-off coefficients:
The percentage of rainfall that appears as storm water run-off
from a surface is called the run-off coefficient.
The run-off coefficient of roofed areas (Cr) is 1.0. The run-off
coefficient of paved areas (Ci) is 0.9. Depending on the soil
type and rainfall intensity the run-off coefficient from pervious
areas (Cp) could be as low as no run-off at all (low rainfall
intensity, sandy soil) or up to 80% (high rainfall, heavy clay
soil).

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You need to know the run-off coefficient to size the storm
water drainage system on the site.
Effects of surface runoff: Erosion and deposition: Surface runoff can
cause erosion of the Earth's surface; eroded material may be
deposited a considerable distance away.
Environmental effects: The principal environmental issues associated
with runoff are the impacts to surface water, groundwater and soil
through transport of water pollutants to these systems.
Agricultural issues: The transport of agricultural chemicals (nitrates,
phosphates, pesticides, herbicides etc.) via surface runoff. The
resulting contaminated runoff represents not only a waste of
agricultural chemicals, but also an environmental threat to
downstream ecosystems.

4 runoff and floods

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